Reaction mass transfer effects

Discussion of the concepts and procedures involved in designing packed gas absorption systems shall first be confined to simple gas absorption processes without compHcations isothermal absorption of a solute from a mixture containing an inert gas into a nonvolatile solvent without chemical reaction. Gas and Hquid are assumed to move through the packing in a plug-flow fashion. Deviations such as nonisotherma1 operation, multicomponent mass transfer effects, and departure from plug flow are treated in later sections. 

Reactor Configuration. The horizontal cross-sectional area of a reactor is a critical parameter with respect to oxygen mass-transfer effects in LPO since it influences the degree of interaction of the two types of zones. Reactions with high intrinsic rates, such as aldehyde oxidations, are largely mass-transfer rate-limited under common operating conditions. Such reactions can be conducted effectively in reactors with small horizontal cross sections. Slower reactions, however, may require larger horizontal cross sections for stable operation. 

Heat transfer and mass transfer occur simultaneously whenever a transfer operation involves a change in phase or a chemical reaction. Of these two situations, only the first is considered herein because in reacting systems the complications of chemical reaction mechanisms and pathways are usually primary (see HeaT-EXCHANGETECHNOLOGy). Even in processes involving phase changes, design is frequendy based on the heat-transfer process alone mass transfer is presumed to add no compHcations. But in fact mass transfer effects do influence and can even limit the process rate. 

When two-phase mass transfer is required to supply reactants by mixing for a chemical reaction, the most important factor to consider is whether the mass transfer controls the operation or whether the chemical reaction controls it. This can be done by increasing the mixer speed to a point w here mass transfer effects become very high and the operation is limited by the chemical reaction. 

The mass transfer effect is relevant when the chemical reaction is far faster than the molecular diffusion, i.e. Ha > 1. The rapid formation of precipitate particles should then occur spatially distributed. The relative rate of particle formation to chemical reaction and/or diffusion can as yet be evaluated only via lengthy calculations. 

Among the earlier studies of reaction kinetics in mechanically stirred slurry reactors may be noted the papers of Davis et al. (D3), Price and Schiewitz (P5), and Littman and Bliss (L6). The latter investigated the hydrogenation of toluene catalyzed by Raney-nickel with a view to establishing the mechanism of the reaction and reaction orders, the study being a typical example of the application of mechanically stirred reactors for investigations of chemical kinetics in the absence of mass-transfer effects. 

In this region, the mass transfer effects are small and the rate is determined almost entirely by the reaction kinetics. 

Kita H, Ye S, Gao Y. 1992. Mass transfer effect in hydrogen evolution reaction on Pt singlecrystal electrodes in acid solution. J Electroanal Chem 334 351-357. 

Table 3 Madon-Boudart test for the absence of mass transfer effects, reaction conditions 45 psig hydrogen / 298 K / 700 RPM.

To illustrate the masking effects that arise from intraparticle and external mass transfer effects, consider a surface reaction whose intrinsic kinetics are second-order in species A. For this rate expression, equation 12.4.20 can be written as... 

This procedure obviously requires machine computation capability if it is to employed in reactor design calculations. Fortunately, there are many reactions for which the global rate reduces to the intrinsic rate, which avoids the necessity for calculations of this type. On the other hand, several high tonnage processes (e.g., S02 oxidation) are influenced by heat and mass transfer effects and one must be fully cognizant of their implications for design purposes. 

The N20 decomposition, CO oxidation, and H2 oxidation reactions are known to exhibit concentration oscillations over noble metal catalysts. Flytzani-Stephanopoulos et al. (47) have observed oscillations for the oxidation of NH3 over Pt. The effects are dramatic and lead to large temperature cycles for the catalyst wire. Heat and mass transfer effects are important. 

Mass transfer effects are very important for the selectivity in the Fischer-Tropsch synthesis. Even though the reactants are in the gas phase, the catalyst pores will be filled with liquid products. Diffusion in the liquid phase is about 3 orders of magnitude slower than in the gas phase and even slow reactions may become diffusion limited. Diffusion limitations may occur through limitation on the arrival of CO to the active points or through the limited removal of reactive products.8,9... 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

An examination of the catalyst-layer models reveals the fact that there are many more cathode models than anode ones. In fact, basically every electrode-only model is for the cathode. This arises because the cathode has the slower reaction it is where water is produced, and hence, mass-transfer effects are much more significant and it represents the principal inefficiency of the fuel cell. In other words, while the cathode model can be separate from the anode model, the converse is not true due to the... 

A factor closely related to the catalyst loading is the efficiency or utilization of the electrode. This tells how much of the electrode is actually being used for electrochemical reaction and can also be seen as a kind of penetration depth. To examine ohmic and mass-transfer effects, sometimes an effectiveness factor, E, is used. This is defined as the actual rate of reaction divided by the rate of reaction without any transport (ionic or reactant) losses. With this introduction of the parameters and equations, the various modeling approaches can be discussed. 

The usual experimental criterion for diffusion control involves an evaluation of the rate of reaction as a function of particle size. At a sufficiently small particle size, the measured rate of reaction will become independent of particle size. The reaction rate can then be safely assumed to be independent of intraparticle mass transfer effects. At the other extreme, if the observed rate is inversely proportional to particle size, the reaction is strongly influenced by intraparticle diffusion. For a reaction whose rate is inhibited by the presence of products, there is an attendant danger of misinterpreting experimental results obtained for different particle sizes when a differential reactor is used, because, under these conditions, the effectiveness factor is sensitive to changes in the partial pressure of product. 

Reactions described by other kinetic routes may be treated in similar fashions. Although, for reasons already explained in Sect. 4.1, mass transfer effects will not influence the selectivity of two concurrent reactions arising from the same reactant, heat transfer between fluid and solid does have an affect. Thus for the first-order reactions... 

We have frnished our discussions of the fundamentals of catalytic reactions, catalytic reactors, and mass transfer effects. While we noted that most catalytic reactions can be made to exhibit complicated kinetics, we have confined our considerations to the almost trivial reaction r" = k"CA- We did this because the algebra was messy enough with the simplest kinetics. 

To ensure the system is probing reactions in a kinetically controlled regime, the reaction conditions must be calculated to determine the value of the Wiesz-Prater criterion. This criterion uses measured values of the rate of reaction to determine if internal dififusion has an influence. Internal mass transfer effects can be neglected for values of the dimensionless number lower than 0.1. For example, taking a measured CPOX rate of 5.9 x 10 molcH4 s g results... 

When a biocatalyst is immobilized on or within a solid matrix, mass transfer effects may exist because the substrate must diffuse from the bulk solution to the immobilized biocatalyst. If the biocatalyst is attached to non-porous supports there are only external mass transfer effects on the catalytically active outer surface in the reaction solution, the supports are surrounded by a stagnant film and substrate and product are transported across this Nemst layer by diffusion. The driving force for this diffusion is the concentration difference between the surface and the bulk concentration of substrate and product. 

Here, issues in relation to the trickle flow regime—isothermal operation and plug flow for the gas phase—will be dealt with. Also, it is assumed that the flowing liquid completely covers the outer surface particles (/w = 1 or aLS = au) so that the reaction can take place solely by the mass transfer of the reactant through the liquid-particle interface. Generally, the assumption of isothermal conditions and complete liquid coverage in trickle-bed processes is fully justified with the exception of very low liquid rates. Capillary forces normally draw the liquid into the pores of the particles. Therefore, the use of liquid-phase diffusivities is adequate in the evaluation of intraparticle mass transfer effects (effectiveness factors) (Smith, 1981). 

The model provides a good approach for the biotransformation system and highlights the main parameters involved. However, prediction of mass transfer effects on the outcome of the process, through evaluation of changes in the mass transfer coefficients, is rather difficult. A similar mass transfer reaction model, but based on the two-film model for mass transfer for a transformation occurring in the bulk aqueous phase as shown in Figure 8.3, could prove quite useful. Each of the films presents a resistance to mass transfer, but concentrations in the two fluids are in equilibrium at the interface, an assumption that holds provided surfactants do not accumulate at the interface and mass transfer rates are extremely high [36]. 

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